Optical modulator and method of manufacturing optical modulator

ABSTRACT

An optical modulator includes a first mesa waveguide extending in a first direction, and a second mesa waveguide. The first mesa waveguide includes a p-type first semiconductor layer disposed over a substrate, a core layer disposed over the first semiconductor layer, a p-type second semiconductor layer disposed over the core layer, and an n-type third semiconductor layer disposed over the core layer. The second semiconductor layer and the third semiconductor layer are arranged adjacent to each other in the first direction. An electrode is disposed over the third semiconductor layer. A joining surface between the second semiconductor layer and the third semiconductor layer is inclined with respect to a surface orthogonal to the first direction.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority based on Japanese Patent ApplicationNo. 2021-098079 filed on Jun. 11, 2021, and the entire contents of theJapanese patent application are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to an optical modulator and a method ofmanufacturing an optical modulator.

BACKGROUND

JP 2004-102160A discloses a Mach-Zehnder optical modulator including twomesa waveguides. Each mesa waveguide includes a lower cladding layerprovided on a substrate, a core layer provided on the lower claddinglayer, and an upper cladding layer provided on the core layer. Aplurality of modulation electrodes arranged in the extending directionof the mesa waveguide are provided on each mesa waveguide. Betweenadjacent modulation electrodes of the plurality of modulationelectrodes, the upper cladding layer has a separating portion made of ani-type semiconductor.

SUMMARY

An optical modulator according to an aspect of the present disclosureincludes a first mesa waveguide extending in a first direction, and asecond mesa waveguide. The first mesa waveguide includes a p-type firstsemiconductor layer disposed over a substrate, a core layer disposedover the first semiconductor layer, a p-type second semiconductor layerdisposed over the core layer, and an n-type third semiconductor layerdisposed over the core layer. The second semiconductor layer and thethird semiconductor layer are arranged adjacent to each other in thefirst direction. An electrode is disposed over the third semiconductorlayer. A joining surface between the second semiconductor layer and thethird semiconductor layer is inclined with respect to a surfaceorthogonal to the first direction.

A method of manufacturing an optical modulator according to an aspect ofthe present disclosure is a method of manufacturing an optical modulatorincluding a first mesa waveguide and a second mesa waveguide extendingin a first direction. The method includes forming a semiconductor stackincluding a first semiconductor layer, a core layer, and a secondsemiconductor layer over a substrate, forming a recess in the secondsemiconductor layer by wet-etching the second semiconductor layer,forming a second conductivity-type third semiconductor layer in therecess, forming the first mesa waveguide and the second mesa waveguideby etching the second semiconductor layer, the third semiconductorlayer, the core layer, and the first semiconductor layer and forming anelectrode over the first mesa waveguide. The first semiconductor stackedlayer is a p-type first semiconductor layer provided over the substrate.The core layer is provided over the first semiconductor layer. Thesecond semiconductor layer is provided over the core layer. The lengthof the recess in the first direction decreases in a direction from anupper end toward a lower end of the recess. The electrode is formed overan n-type layer selected from the second semiconductor layer and thethird semiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other purposes, aspects and advantages will be betterunderstood from the following detailed description with reference to thedrawings.

FIG. 1 is a plan view schematically illustrating an optical modulatoraccording to a first embodiment.

FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1 .

FIG. 3 is a cross-sectional view taken along line of FIG. 1 .

FIG. 4 is a cross-sectional view taken along line IV-IV of FIG. 1 .

FIG. 5 is a graph showing a relationship between depth and dopantconcentration in a first experiment.

FIG. 6 is a graph showing a relationship between depth and dopantconcentration in a second experiment.

FIG. 7 is a graph showing an example of a relationship between theposition in the waveguide direction and the electric field intensity.

FIG. 8 is a graph showing an example of a relationship between acceptorconcentration and electric field leakage length.

FIG. 9 is a plan view schematically illustrating one step of the methodof manufacturing an optical modulator according to the first embodiment.

FIG. 10 is a cross-sectional view taken along line X-X of FIG. 9 .

FIG. 11 is a plan view schematically illustrating one step of the methodof manufacturing the optical modulator according to the firstembodiment.

FIG. 12 is a cross-sectional view taken along line XII-XII of FIG. 11 .

FIG. 13 is a plan view schematically illustrating one step of the methodof manufacturing the optical modulator according to the firstembodiment.

FIG. 14 is a cross-sectional view taken along line XIV-XIV of FIG. 13 .

FIG. 15 is a cross-sectional view schematically illustrating one step ofthe method of manufacturing the optical modulator according to the firstembodiment.

FIG. 16 is a cross-sectional view schematically illustrating a part ofthe optical modulator according to the second embodiment.

DETAILED DESCRIPTION

When the lower cladding layer is formed of a p-type semiconductor, ap-type dopant (for example, zinc) contained in the lower cladding layerand a dopant (for example, iron) contained in the separating portion mayinterdiffuse. As a result, the p-type dopant diffuses into the corelayer. The p-type dopant in the core may be a non-radiativerecombination center.

The present disclosure provides an optical modulator capable of reducinga p-type dopant concentration in a core layer, and a method ofmanufacturing the optical modulator.

Description of Embodiments of the Present Disclosure

An optical modulator according to an embodiment includes a first mesawaveguide extending in a first direction, and a second mesa waveguide.The first mesa waveguide includes a p-type first semiconductor layerdisposed over a substrate, a core layer disposed over the firstsemiconductor layer, a p-type second semiconductor layer disposed overthe core layer, and an n-type third semiconductor layer disposed overthe core layer. The second semiconductor layer and the thirdsemiconductor layer are arranged adjacent to each other in the firstdirection. An electrode is disposed over the third semiconductor layer.A joining surface between the second semiconductor layer and the thirdsemiconductor layer is inclined with respect to a surface orthogonal tothe first direction.

According to the optical modulator of this embodiment, it is less likelyfor the p-type dopant in the first semiconductor layer and the p-typedopant in the second semiconductor layer to mutually diffuse. Therefore,it is possible to suppress diffusion of the p-type dopant in the firstsemiconductor layer and the second semiconductor layer toward the corelayer. Therefore, the p-type dopant concentration of the core layer canbe reduced.

The joining surface may be inclined such that a length of the thirdsemiconductor layer in the first direction is decreased in a directionfrom the electrode toward the core layer. In this case, even if anelectric field leakage extending from the joining surface into thep-type second semiconductor layer in the first direction occurs, theregion in which the electric field spreads in the first direction can bereduced.

The p-type dopant in the second semiconductor layer may have aconcentration of 1×10¹⁶ cm⁻³ or more. In this case, the electric fieldleakage length extending from the joining surface into the p-type secondsemiconductor layer in the first direction can be reduced.

The p-type dopant in the second semiconductor layer may have aconcentration of 1×10¹⁷ cm⁻³ or less. In this case, optical loss due tofree carrier absorption can be reduced in the second semiconductorlayer.

A method of manufacturing an optical modulator according to anembodiment is a method of manufacturing an optical modulator including afirst mesa waveguide and a second mesa waveguide extending in a firstdirection. The method includes forming a semiconductor stack including afirst semiconductor layer, a core layer, and a second semiconductorlayer over a substrate, forming a recess in the second semiconductorlayer by wet-etching the second semiconductor layer, forming a secondconductivity-type third semiconductor layer in the recess, forming thefirst mesa waveguide and the second mesa waveguide by etching the secondsemiconductor layer, the third semiconductor layer, the core layer, andthe first semiconductor layer and forming an electrode over the firstmesa waveguide. The first semiconductor stacked layer is a p-type firstsemiconductor layer provided over the substrate. The core layer isprovided over the first semiconductor layer. The second semiconductorlayer is provided over the core layer. The length of the recess in thefirst direction decreases in a direction from an upper end toward alower end of the recess. The electrode is formed over an n-type layerselected from the second semiconductor layer and the third semiconductorlayer.

According to the method of manufacturing an optical modulator of thepresent embodiment, the p-type dopant in the first semiconductor layerand the p-type dopant in the p-type layer selected from the secondsemiconductor layer and the third semiconductor layer are less likely tointerdiffuse. Therefore, it is possible to suppress diffusion of thep-type dopant in the first semiconductor layer and the secondsemiconductor layer or the third semiconductor layer toward the corelayer. Therefore, the p-type dopant concentration of the core layer canbe reduced.

The third semiconductor layer may be the n-type layer. In this case, itis less likely for the p-type dopant in the p-type second semiconductorlayer to diffuse toward the core layer.

In the forming a third semiconductor layer, a length of an upper surfaceof the third semiconductor layer in the first direction may be largerthan a length of an upper surface of the second semiconductor layer inthe first direction. In this case, the flatness of the upper surface ofthe third semiconductor layer can be improved.

Details of Embodiment of the Present Disclosure

Hereinafter, embodiments of the present disclosure will be described indetail with reference to the accompanying drawings. In the descriptionof the drawings, the identical reference numerals are used for the sameor equivalent elements, and redundant description is omitted. In thedrawings, an X-axis direction, a Y-axis direction, and a Z-axisdirection that intersect each other are illustrated as necessary. TheX-axis direction, the Y-axis direction, and the Z-axis direction areorthogonal to each other, for example. The X-axis direction is, forexample, a [0-11] direction. The Y-axis direction is, for example, a[011] direction. The Z-axis direction is, for example, a [100]direction.

In the present disclosure, the term “on” or “over” is not limited tomean that a component is in contact with another component. For example,when the first layer is disposed on or over the second layer, the firstlayer and the second layer may be in contact with each other or one ormore layers may be disposed between the first layer and the secondlayer.

First Embodiment

FIG. 1 is a plan view schematically illustrating an optical modulatoraccording to a first embodiment. An optical modulator 10 illustrated inFIG. 1 is, for example, a Mach-Zehnder modulator. Optical modulator 10can modulate the intensity or phase of light in optical communication,for example, and generate a modulation signal. Optical modulator 10 canattenuate the light by adjusting the intensity of the light, forexample.

Optical modulator 10 includes a first arm waveguide AM1 and a second armwaveguide AM2 of a Mach-Zehnder modulator. First arm waveguide AM1includes a first mesa waveguide M1. First arm waveguide AM1 may includea pair of bent waveguides M1 b optically coupled to both ends of firstmesa waveguide M1, respectively. Second arm waveguide AM2 includes asecond mesa waveguide M2. Second arm waveguide AM2 may include a pair ofbent waveguides M2 b optically coupled to both ends of second mesawaveguide M2, respectively.

First mesa waveguide M1 is provided over a substrate 12, extends alongthe Y-axis direction (first direction), and has a height in the Z-axisdirection. Second mesa waveguide M2 extends along first mesa waveguideM1. Second mesa waveguide M2 may extend along the Y-axis direction andmay have a height in the Z-axis direction. The extending direction ofsecond mesa waveguide M2 may be different from the extending directionof first mesa waveguide M1. Specifically, the angle formed by theextending direction of first mesa waveguide M1 and the extendingdirection of second mesa waveguide M2 may be, for example, more than 0°and 20° or less. First mesa waveguide M1 and second mesa waveguide M2are separated from each other in the X-axis direction.

An input end of first mesa waveguide M1 and an input end of second mesawaveguide M2 are optically coupled to an optical demultiplexer C1 viabent waveguide M1 b and bent waveguide M2 b, respectively. Opticaldemultiplexer C1 is, for example, a multi-mode interference (MMI)coupler such as a 1×2 multi-mode interference coupler. Opticaldemultiplexer C1 is optically coupled to the output end of an inputwaveguide W1. The input end of input waveguide W1 is an input port P1.Input port P1 is located at the edge of substrate 12. The light is inputto input port P1.

The output end of first mesa waveguide M1 and the output end of secondmesa waveguide M2 are optically coupled to an optical multiplexer C2 viabent waveguide M1 b and bent waveguide M2 b, respectively. Opticalmultiplexer C2 is, for example, an MMI coupler such as a 2×1 multimodeinterference coupler. Optical multiplexer C2 is optically coupled to aninput end of an output waveguide W2. An output end of output waveguideW2 is an output port P2. Output port P2 is located at an edge oppositeto the edge of substrate 12 where input port P1 is located. Light isoutput from output port P2.

First mesa waveguide M1 includes a plurality of modulating portions M1 mspaced apart from each other along the Y-axis direction. A separatingportion M1 s is located between adjacent modulating portions of theplurality of modulating portions M1 m. A wiring line E1 a extending inthe Y-axis direction is connected to each modulating portion M1 m.Wiring line E1 a is located over modulating portion M1 m. Each wiringline E1 a is connected to an electrode pad EP1 by a wiring line E1 b.Electrode pad EP1 is located away from wiring line E1 a in the X-axisdirection. Electrode pad EP1 extends in the Y-axis direction over theplurality of modulating portions M1 m. Wiring line E1 a, wiring line E1b, and electrode pad EP1 are located over substrate 12. Wiring line E1a, wiring line E1 b, and electrode pad EP1 include metals such as gold.

Second mesa waveguide M2 has the same configuration as first mesawaveguide M1. Second mesa waveguide M2 includes a plurality ofmodulating portions M2 m spaced apart from each other along the Y-axisdirection. A separating portion M2 s is located between adjacentmodulation portions of the plurality of modulating portions M2 m. Awiring line E2 a extending in the Y-axis direction is connected to eachmodulating portion M2 m. A wiring line E2 a is located over modulatingportion M2 m. Each wiring line E2 a is connected to an electrode pad EP2by a wiring line E2 b. Electrode pad EP2 is located away from wiringline E2 a in the X-axis direction. Electrode pad EP2 extends in theY-axis direction over the plurality of modulating portions M2 m. Wiringline E2 a, wiring line E2 b, and electrode pad EP2 are located oversubstrate 12. Wiring line E2 a, wiring line E2 b, and electrode pad EP2include metals such as gold.

A drive circuit DR is connected to one end of electrode pad EP1 and oneend of electrode pad EP2 by a wiring line. Drive circuit DR includes analternating-current power supply PW, a resistor R1, and a resistor R2.Alternating-current power supply PW is connected to one end of electrodepad EP1 through resistor R1 by a wiring line. Alternating-current powersupply PW is connected to one end of electrode pad EP2 through resistorR2 by a wiring line.

The other end of electrode pad EP1 is connected to a ground potentialGND via a termination resistor RT1 by a wiring line. The other end ofelectrode pad EP2 is connected to ground potential GND via a terminationresistor RT2 by a wiring line.

FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1 . FIG.2 illustrates a cross-section of modulating portion M1 m and modulatingportion M2 m. FIG. 3 is a cross-sectional view taken along line of FIG.1 . FIG. 3 illustrates a cross section of separating portion M1 s andseparating portion M2 s. FIG. 4 is a cross-sectional view taken alongline IV-IV of FIG. 1 . FIG. 4 illustrates a cross section of modulatingportion M1 m and separating portion M1 s.

As illustrated in FIG. 2 to FIG. 4 , first mesa waveguide M1 includes ap-type first semiconductor layer 16 provided over substrate 12, a corelayer 18 provided over first semiconductor layer 16, a p-type secondsemiconductor layer 26 provided over core layer 18, and an n-type thirdsemiconductor layer 20 provided over core layer 18. A semiconductorlayer 14 may be provided between substrate 12 and first semiconductorlayer 16. A p-type semiconductor layer 28 may be provided over secondsemiconductor layer 26. An n-type semiconductor layer 22 may be providedover third semiconductor layer 20.

Second semiconductor layer 26 and third semiconductor layer 20 aredisposed adjacent to each other in the Y-axis direction. The pluralityof second semiconductor layers 26 and the plurality of thirdsemiconductor layers 20 may be arranged in the Y-axis direction. Secondsemiconductor layers 26 and third semiconductor layers 20 may bealternately arranged in the Y-axis direction. The number of thirdsemiconductor layers 20 may correspond to the number of electrodes E1. Ajoining surface JN between second semiconductor layer 26 and thirdsemiconductor layer 20 is inclined with respect to a plane (XZ plane)orthogonal to the Y-axis direction. That is, joining surface JN isinclined with respect to the Z-axis direction in the YZ cross section offirst mesa waveguide M1. Joining surface JN is a surface of PN junction.In the present embodiment, joining surface JN is inclined such that thelength L20 of third semiconductor layer 20 in the Y-axis directionbecomes shorter from electrode E1 toward core layer 18. That is, joiningsurface JN is inclined such that the length L26 of second semiconductorlayer 26 in the Y-axis direction increases from electrode E1 toward corelayer 18. Joining surface JN is inclined by an angle (90°-θ) withrespect to the XZ plane. That is, joining surface JN is inclined by theangle θ with respect to a main surface 12 a of substrate 12. The angle θmay be from 50° to 60°. The angle θ can be controlled by the crystalorientation of the group III-V compound semiconductor included in secondsemiconductor layer 26. For example, when main surface 12 a of substrate12 is a (100) plane and second semiconductor layer 26 is epitaxiallygrown over main surface 12 a, the angle θ may be 55°.

Joining surface JN may be inclined such that the length L20 of thirdsemiconductor layer 20 in the Y-axis direction increases from electrodeE1 toward core layer 18. That is, joining surface JN may be inclinedsuch that a length L26 of second semiconductor layer 26 in the Y-axisdirection becomes shorter from electrode E1 toward core layer 18.

In first mesa waveguide M1, electrode E1 is provided over thirdsemiconductor layer 20. Semiconductor layer 22 may be disposed betweenthird semiconductor layer 20 and electrode E1. Wiring line E1 a isprovided over electrode E1. In first mesa waveguide M1, electrode E1 andwiring line E1 a are not provided over second semiconductor layer 26.

Similarly, in second mesa waveguide M2, an electrode E2 is provided overthird semiconductor layer 20. Semiconductor layer 22 may be disposedbetween third semiconductor layer 20 and electrode E2. Wiring line E2 ais provided over electrode E2. In second mesa waveguide M2, electrode E2and wiring line E2 a are not provided over second semiconductor layer26.

As shown in FIG. 2 , in modulating portion M1 m and modulating portionM2 m, semiconductor layer 14, first semiconductor layer 16, core layer18, third semiconductor layer 20, and semiconductor layer 22 may besequentially provided over main surface 12 a of substrate 12. Firstsemiconductor layer 16 constitutes a lower cladding layer. Thirdsemiconductor layer 20 constitutes an upper cladding layer. Core layer18 of first mesa waveguide M1 and core layer 18 of second mesa waveguideM2 are arranged to be spaced apart from each other in the X-axisdirection. In the cross section of first mesa waveguide M1 orthogonal tothe Y-axis direction, a spot S1 of light is formed over firstsemiconductor layer 16, core layer 18, and third semiconductor layer 20.In the cross section of second mesa waveguide M2 orthogonal to theY-axis direction, a spot S2 of light is formed over first semiconductorlayer 16, core layer 18, and third semiconductor layer 20.

As shown in FIG. 3 , in separating portion M1 s and separating portionM2 s, semiconductor layer 14, first semiconductor layer 16, core layer18, second semiconductor layer 26, and semiconductor layer 28 may besequentially provided over main surface 12 a of substrate 12. Secondsemiconductor layer 26 constitutes an upper cladding layer.

Substrate 12 is, for example, a semi-insulating semiconductor substrate.Substrate 12 includes a group III-V compound semiconductor doped with aninsulating dopant. Substrate 12 includes, for example, InP doped withiron (Fe). The dopant concentration of substrate 12 may be from 1×10¹⁷cm⁻³ to 1×10¹⁸ cm⁻³.

As shown in FIG. 2 and FIG. 3 , semiconductor layer 14 includes a firstportion 14 a located between core layer 18 and substrate 12 and a pairof second portions 14 b located on both sides of first portion 14 a.First portion 14 a and pair of second portions 14 b extend in the Y-axisdirection. Therefore, the width (length in the X-axis direction) ofsemiconductor layer 14 is larger than the width of core layer 18.Semiconductor layer 14 of first mesa waveguide M1 and semiconductorlayer 14 of second mesa waveguide M2 are connected to each other. In thepresent embodiment, semiconductor layer 14 of first mesa waveguide M1and semiconductor layer 14 of second mesa waveguide M2 are connected toeach other to form a single semiconductor layer. Semiconductor layer 14may not include the pair of second portions 14 b. In this case,semiconductor layer 14 of first mesa waveguide M1 and semiconductorlayer 14 of second mesa waveguide M2 can be electrically connected toeach other by a semiconductor layer or a conductive layer providedbetween substrate 12 and semiconductor layer 14.

Semiconductor layer 14 may include a group III-V compound semiconductordoped with a p-type dopant. Semiconductor layer 14 includes, forexample, InGaAs or InP doped with zinc (Zn). Semiconductor layer 14 hasa dopant concentration greater than the dopant concentration of firstsemiconductor layer 16. The dopant concentration of semiconductor layer14 may be ten times or more the dopant concentration of firstsemiconductor layer 16. The dopant concentration of semiconductor layer14 may be 5×10¹⁸ cm⁻³ or more or 1×10¹⁹ cm⁻³ or more. A thickness T1 ofsemiconductor layer 14 is, for example, from 0.5 μm to 2.0 μm.

First semiconductor layer 16 includes a first portion 16 a locatedbetween core layer 18 and semiconductor layer 14 and a pair of secondportions 16 b located on both sides of first portion 16 a. The thicknessof first portion 16 a is greater than the thickness of second portion 16b. First portion 16 a and pair of second portions 16 b extend in theY-axis direction. Therefore, the width of first semiconductor layer 16is larger than the width of core layer 18. First semiconductor layer 16of first mesa waveguide M1 and first semiconductor layer 16 of secondmesa waveguide M2 are connected to each other. In the presentembodiment, first semiconductor layer 16 of first mesa waveguide M1 andfirst semiconductor layer 16 of second mesa waveguide M2 are connectedto each other to form a single semiconductor layer. First semiconductorlayer 16 may not include the pair of second portions 16 b.

First semiconductor layer 16 includes a group III-V compoundsemiconductor doped with a p-type dopant. First semiconductor layer 16may include a semiconductor material different from the semiconductormaterial of semiconductor layer 14. First semiconductor layer 16includes, for example, InP doped with Zn. The dopant concentration offirst semiconductor layer 16 may be from 1×10¹⁷ cm⁻³ to 2×10¹⁸ cm⁻³. Thethickness T2 of first semiconductor layer 16 (thickness of first portion16 a) may be larger than the thickness T1 of semiconductor layer 14, andmay be, for example, from 1.0 μm to 3.0 μm.

Core layer 18 is an i-type semiconductor layer, that is, an undopedsemiconductor layer. Core layer 18 may have a multiple quantum wellstructure. Core layer 18 includes, for example, an AlGaInAs-based groupIII-V compound semiconductor. The width of core layer 18 is, forexample, 1.5 μm or less. The p-type dopant concentration in core layer18 is, for example, 1×10¹⁶ cm⁻³ or less.

Third semiconductor layer 20 includes a group III-V compoundsemiconductor doped with an n-type dopant. Third semiconductor layer 20includes, for example, InP doped with Si. The dopant concentration ofthird semiconductor layer 20 may be from 1×10¹⁷ cm⁻³ to 2×10¹⁸ cm⁻³. Thethickness of third semiconductor layer 20 is, for example, from 1.0 μmto 3.0 μm.

Semiconductor layer 22 includes a group III-V compound semiconductordoped with an n-type dopant. Semiconductor layer 22 may include asemiconductor material different from the semiconductor material ofthird semiconductor layer 20. Semiconductor layer 22 includes, forexample, InGaAs or InP doped with Si. Semiconductor layer 22 has adopant concentration greater than that of third semiconductor layer 20.The dopant concentration of semiconductor layer 22 may be 1×10¹⁸ cm⁻³ ormore or 1×10¹⁹ cm⁻³ or more. The thickness of semiconductor layer 22 is,for example, from 0.1 μm to 0.5 μm.

Second semiconductor layer 26 includes a group III-V compoundsemiconductor doped with a p-type dopant. Second semiconductor layer 26may include the same semiconductor material as the semiconductormaterial of first semiconductor layer 16. Second semiconductor layer 26includes, for example, InP doped with Zn. Second semiconductor layer 26has a dopant concentration smaller than that of first semiconductorlayer 16. The dopant concentration of second semiconductor layer 26 maybe from 5×10¹⁶ cm⁻³ to 5×10¹⁷ cm⁻³, or from 5×10¹⁶ cm⁻³ to 1×10¹⁷ cm⁻³.Second semiconductor layer 26 may be thinner than T2 of firstsemiconductor layer 16, and may be, for example, from 1.0 μm to 3.0 μm.

Semiconductor layer 28 may include a group III-V compound semiconductordoped with a p-type dopant. Semiconductor layer 28 may include asemiconductor material different from the semiconductor material ofsecond semiconductor layer 26. Semiconductor layer 28 includes, forexample, InGaAs or InP doped with Zn. Semiconductor layer 28 has adopant concentration smaller than that of first semiconductor layer 16.The dopant concentration of semiconductor layer 28 may be from 5×10¹⁶cm⁻³ to 5×10¹⁷ cm⁻³, or from 5×10¹⁶ cm⁻³ to 1×10¹⁷ cm⁻³. The thicknessof 10¹⁶ Semiconductor layer 28 is, for example, from 10 nm to 100 nm.

Electrode E1 is connected to semiconductor layer 22 of first mesawaveguide M1. Electrode E1 is in ohmic contact with semiconductor layer22 of first mesa waveguide M1. Electrode E1 is connected to wiring lineE1 a. Similarly, electrode E2 is connected to semiconductor layer 22 ofsecond mesa waveguide M2. Electrode E2 is in ohmic contact withsemiconductor layer 22 of second mesa waveguide M2. Electrode E2 isconnected to wiring line E2 a. Each of electrodes E1 and E2 includes,for example, a Ni layer, a Ge layer, and an Au layer. A furtherelectrode may be connected to semiconductor layer 14.

An insulating film 30 including, for example, an inorganic material maybe provided over main surface 12 a of substrate 12, the side surface offirst mesa waveguide M1, and the side surface of second mesa waveguideM2. An embedded region 32 may be provided over insulating film 30 toembed first mesa waveguide M1 and second mesa waveguide M2. Embeddedregion 32 includes, for example, resin. Insulating film 30 may beprovided over embedded region 32.

In optical modulator 10 of the present embodiment, an AC voltage isapplied to electrode E1 and electrode E2 by drive circuit DR. Forexample, by applying a voltage to first mesa waveguide M1, the intensityor phase of the light propagating through core layer 18 of first mesawaveguide M1 is adjusted. Similarly, by applying a voltage to secondmesa waveguide M2, the intensity or phase of the light propagatingthrough core layer 18 of second mesa waveguide M2 is adjusted.

According to optical modulator 10 of the present embodiment, it is lesslikely for the p-type dopant in second semiconductor layer 26 and thep-type dopant in first semiconductor layer 16 to mutually diffuse.Therefore, diffusion of the p-type dopant in first semiconductor layer16 and second semiconductor layer 26 toward core layer 18 can besuppressed. Therefore, the p-type dopant concentration of core layer 18can be reduced.

As illustrated in FIG. 4 , joining surface JN may be inclined such thatthe length L20 of third semiconductor layer 20 in the Y-axis directiondecreases from electrode E1 toward core layer 18. In this case, even ifan electric field leakage extending from the joining surface JN into thep-type second semiconductor layer 26 in the Y-axis direction occurs, theregion in which the electric field spreads in the Y-axis direction canbe reduced. Therefore, the length in the Y-axis direction of the regionto which the voltage is applied in core layer 18 can be reduced.

When the p-type dopant in second semiconductor layer 26 has aconcentration of 1×10¹⁶ cm⁻³ or more, the electric field leakageextending from joining surface JN into the p-type second semiconductorlayer 26 in the Y-axis direction can be reduced. When the p-type dopantin second semiconductor layer 26 has a concentration of 1×10¹⁷ cm⁻³ orless, optical loss due to absorption of free carriers can be reduced insecond semiconductor layer 26.

Hereinafter, various experiments performed to evaluate optical modulator10 will be described. The experiments described below are not intendedto limit the present disclosure.

(First Experiment)

The optical modulator of the first experiment has a structureillustrated in FIG. 1 to FIG. 4 . Specifically, the optical modulator ofthe first experiment has the following structures:

Substrate 12:

InP substrate doped with Fe (Fe concentration: from 1×10¹⁷ cm⁻³ to1×10¹⁸ cm⁻³),semiconductor layer 14: p-InGaAs contact layer,Zn-doped InGaAs layer (1.1 μm thick, Zn concentration 2×10¹⁹ cm⁻³),first semiconductor layer 16: p-InP lower cladding layer,InP layer doped with Zn (first portion 16 a is 1.5 μm thick, secondportion 16 b is 1 μm thick, and Znconcentration is from 5×10¹⁷ cm⁻³ to 2×10¹⁸ cm⁻³),core layer 18: i-core layer,AlGaInAs/AlInAs multiple quantum wells (0.5 μm thick, 1.5 μm wide, and15 μm between first mesa waveguide M1 core layer 18 and second mesawaveguide M2 core layer 18),third semiconductor layer 20:Si-doped InP layer (Si-concentration from 5×10¹⁷ cm⁻³ to 2×10¹⁸ cm⁻³),semiconductor layer 22:Si-doped InGaAs layer (Si-concentration 1×10¹⁹ cm⁻³ or more),second semiconductor layer 26: p-InP upper cladding layer,InP layer doped with Zn (1.2 μm thick, Zn concentration from 1×10¹⁷ cm⁻³to 5×10¹⁷ cm⁻³), andsemiconductor layer 28: p-InGaAs layer,Zn-doped InGaAs layer (50 nm thick, Zn concentration from 1×10¹⁷ cm⁻³ to5×10¹⁷ cm⁻³).

SIMS (Secondary Ion Mass Spectrometry) measurement was performed on theoptical modulator of the first experiment. The results are shown in FIG.5 .

FIG. 5 is a graph showing the relationship between depth and dopantconcentration in the first experiment. In the graph, the vertical axisrepresents the dopant concentration. The dopant concentration is the Znconcentration. The horizontal axis represents the depth in a directionperpendicular to the main surface of the InP substrate. The position atwhich the depth becomes zero is the upper surface of the p-InGaAs layer.The graph shows a profile P1ZN of the Zn concentration. In the i-corelayer, the Zn concentration was as small as 1×10¹⁶ cm⁻³ or less. It canbe seen from the graph of FIG. 5 that Zn in the p-InP lower claddinglayer and Zn in the p-InP upper cladding layer do not diffuse into thei-core layer.

(Second Experiment)

The optical modulator of the second experiment has the same structure asthe optical modulator of the first experiment except that an SI(Semi-Insulating)-InP upper cladding layer and an SI-InGaAsP layer areprovided instead of the p-InP upper cladding layer and the p-InGaAslayer, respectively. Each of the SI-InP upper cladding layer and theSI-InGaAsP layer contains Fe as a dopant.

SIMS measurement was performed on the optical modulator of the secondexperiment. The results are shown in FIG. 6 .

FIG. 6 is a graph showing the relationship between the depth and thedopant concentration in the second experiment. In the graph, thevertical axis represents the dopant concentration. The dopantconcentration is the Zn concentration or the Fe concentration. Thehorizontal axis represents the depth in a direction perpendicular to themain surface of the InP substrate. The position at which the depthbecomes zero is the upper surface of the SI-InGaAsP layer. In the graph,a profile P2ZN of the Zn concentration and a profile P2FE of the Feconcentration are illustrated. In the core layer, the Zn concentrationwas greater than 1×10¹⁶ cm⁻³. From the graph of FIG. 6 , it can be seenthat Zn in the p-InP lower cladding layer diffuses into the i-corelayer, and Fe in the SI-InP upper cladding layer diffuses into the p-InPlower cladding layer.

(Third Experiment)

In the third experiment, the electric field intensity distribution wascalculated by simulation for the model structure corresponding to theregion including second semiconductor layer 26 and the pair of thirdsemiconductor layers 20 located on both sides of second semiconductorlayer 26 in FIG. 4 . The results are shown in FIG. 7 .

FIG. 7 is a graph showing an example of the relationship between theposition in the waveguide direction and the electric field intensity. Inthe graph, the vertical axis represents electric field intensity. Thehorizontal axis indicates the position in the waveguide direction(Y-axis direction). The model structure includes a pair of n-InP layerseach having a length of 1 μm in the waveguide direction and a p-InPlayer having a length of 5 μm in the waveguide direction. The Siconcentration in each n-InP layer is 2×10¹⁸ cm⁻³. The acceptorconcentration (Zn concentration) of the p-InP layer is 1×10¹⁴ cm⁻³,1×10¹⁵ cm⁻³, or 1×10¹⁶ cm⁻³. The joining surface between each n-InPlayer and p-InP layer is perpendicular to the waveguiding direction.

In the graph, a profile C14 indicates the electric field intensitydistribution in the p-InP layer when the accepter concentration is1×10¹⁴ cm⁻³. A profile C15 indicates an electric field intensitydistribution in the p-InP layer when the accepter concentration is1×10¹⁵ cm⁻³. A profile C16 shows the electric field intensitydistribution in the p-InP layer when the accepter concentration is1×10¹⁶ cm⁻³. When the concentration of acceptors is 1×10¹⁵ cm⁻³, anelectric field leakage length L15 indicates the length in the waveguidedirection of a region having an electric field intensity of 1% or morewith respect to the peak of the electric field intensity. When theconcentration of acceptors is 1×10¹⁶ cm⁻³, an electric field leakagelength L16 indicates the length in the waveguide direction of a regionhaving an electric field intensity of 1% or more with respect to thepeak of the electric field intensity. It can be seen from the graph ofFIG. 7 that the electric field leakage length becomes shorter as theacceptor concentration becomes higher.

FIG. 8 is a graph showing an example of the relationship between theacceptor concentration and the electric field leakage length. In thegraph, the vertical axis represents the electric field leakage length.The horizontal axis represents acceptor concentration. Electric fieldleakage L15 is about 3 μm. Electric field leakage L16 is about 1 μm. Itcan be seen from the graph of FIG. 8 that the electric field leakagelength linearly decreases as the acceptor concentration increases. Whenthe acceptor concentration is 1×10¹⁶ cm⁻³ or more, the electric fieldleakage is as small as 1 μm or less.

Hereinafter, a method of manufacturing an optical modulator according toa first embodiment will be described with reference to FIG. 9 to FIG. 15. Optical modulator 10 can be manufactured as follows.

(Formation of Semiconductor Stack)

First, as shown in FIG. 9 and FIG. 10 , a semiconductor stack SL isformed over substrate 12. Semiconductor stack SL includes a p-type firstsemiconductor layer 16 provided over substrate 12, core layer 18provided over first semiconductor layer 16, and a p-type (firstconductivity type) second semiconductor layer 26 provided over corelayer 18. Semiconductor stack SL may include semiconductor layer 14disposed between substrate 12 and first semiconductor layer 16.Semiconductor stack SL may include a p-type semiconductor layer 28provided over second semiconductor layer 26. Each layer is formed by,for example, organometallic vapor phase epitaxy (OMVPE).

Next, a mask MK1 is formed over semiconductor stack SL. In the presentembodiment, a plurality of masks MK1 extend in the X-axis direction andare arranged to be spaced apart from each other in the Y-axis direction.Mask MK1 may be formed by, for example, photolithography and etching.

(Wet-Etching of Second Semiconductor Layer)

Next, as shown in FIG. 11 and FIG. 12 , a recess RS is formed in secondsemiconductor layer 26 by wet-etching second semiconductor layer 26.Semiconductor layer 28 may also be wet-etched. Wet-etching is performedusing mask MK1. Examples of etchants include aqueous solutionscomprising hydrochloric acid and hydrogen peroxide. The length LRS ofrecess RS in the Y-axis direction decreases from an upper end RS1 towarda lower end RS2 of recess RS. That is, the side wall of recess RS has aforward tapered shape.

(Formation of Third Semiconductor Layer)

Next, as shown in FIG. 11 and FIG. 12 , an n-type (second conductivitytype opposite to the first conductivity type) third semiconductor layer20 is formed in recess RS. Thereafter, semiconductor layer 22 may beformed over third semiconductor layer 20. Each layer is formed by metalorganic chemical vapor deposition using, for example, mask MK1. Eachlayer may not be formed over mask MK1 by selective growth.

The length L20U of the upper surface of third semiconductor layer 20 inthe Y-axis direction may be larger than the length L26U of the uppersurface of second semiconductor layer 26 in the Y-axis direction. Inthis case, the flatness of the upper surface of third semiconductorlayer 20 can be improved.

(Formation of First Mesa Waveguide and Second Mesa Waveguide)

Next, as shown in FIG. 13 to FIG. 15 , first mesa waveguide M1 andsecond mesa waveguide M2 are formed by etching second semiconductorlayer 26, third semiconductor layer 20, core layer 18, and firstsemiconductor layer 16. Semiconductor layer 22 and semiconductor layer28 may also be etched. First mesa waveguide M1 and second mesa waveguideM2 may be formed by photolithography and dry etching. First, as shown inFIG. 13 and FIG. 14 , a mask MK2 is used while etching secondsemiconductor layer 26, semiconductor layer 28, third semiconductorlayer 20, semiconductor layer 22, core layer 18, and first semiconductorlayer 16. First semiconductor layer 16 is partially etched. In thepresent embodiment, the pair of masks MK2 extend in the Y-axis directionand are arranged to be separated from each other in the X-axisdirection. Next, as shown in FIG. 15 , first semiconductor layer 16 andsemiconductor layer 14 are etched using a mask MK3 extending in theY-axis direction. The width of mask MK3 in the X-axis direction islarger than the width of mask MK2 in the X-axis direction. Thus, firstmesa waveguide M1 and second mesa waveguide M2 are formed.

Next, as shown in FIG. 2 and FIG. 3 , insulating film 30 is formed tocover first mesa waveguide M1 and second mesa waveguide M2. Then, aresin is coated over insulating film 30 to form embedded region 32.Then, insulating film 30 is formed over embedded region 32.

(Formation of Electrode)

Next, as shown in FIG. 2 and FIG. 4 , electrode E1 is formed over firstmesa waveguide M1. Electrode E1 is formed over the n-type thirdsemiconductor layer 20 and is not formed over the p-type secondsemiconductor layer 26. Electrode E1 may be formed over semiconductorlayer 22. Similarly, as shown in FIG. 2 , electrode E2 is formed oversecond mesa waveguide M2. Electrode E2 is formed over the n-type thirdsemiconductor layer 20 and is not formed over the p-type secondsemiconductor layer 26. Electrode E2 may be formed over semiconductorlayer 22.

Next, wiring line E1 a and wiring line E2 a are formed over electrode E1and electrode E2, respectively. At the same time, wiring line E1 b,wiring line E2 b, electrode pad EP1 and electrode pad EP2 are alsoformed. The wiring line and the electrode pad may be formed by, forexample, photolithography, dry etching, vapor deposition, and lift-off.

In the manufacturing method described above, the order of forming secondsemiconductor layer 26 and third semiconductor layer 20 may be reversed.In this case, in the forming the semiconductor stack, semiconductorstack SL includes an n-type (first conductivity type) thirdsemiconductor layer 20 (second semiconductor layer) provided over corelayer 18. In the wet-etching the second semiconductor layer, a recess isformed in third semiconductor layer 20 by wet-etching thirdsemiconductor layer 20. In the forming the third semiconductor layer, ap-type (second conductivity type) second semiconductor layer 26 isformed in the recess. In this case, joining surface JN shown in FIG. 4is inclined such that the length L20 of third semiconductor layer 20 inthe Y-axis direction increases from electrode E1 toward core layer 18.

Second Embodiment

FIG. 16 is a cross-sectional view schematically illustrating a part ofthe optical modulator according to the second embodiment. The opticalmodulator shown in FIG. 16 has the same configuration as that of opticalmodulator 10 of the first embodiment except that it further includes anetching stop layer 50.

Etching stop layer 50 may function as an etching stop layer whenwet-etching second semiconductor layer 26. Etching stop layer 50includes a group III-V compound semiconductor such as InGaAsP.

In this embodiment, the same effects as those of the first embodimentcan be obtained. Etching stop layer 50 can prevent core layer 18 frombeing etched when wet-etching second semiconductor layer 26 or thirdsemiconductor layer 20.

Although the preferred embodiments of the present disclosure have beendescribed in detail above, the present disclosure is not limited to theabove embodiments. The constituent elements of the embodiments may bearbitrarily combined.

The embodiments disclosed herein are to be considered in all respects asillustrative and not restrictive. The scope of the present invention isnot limited by the above-described embodiments but is defined by theclaims, and is intended to include meanings equivalent to the scope ofclaims and all modifications within the scope.

What is claimed is:
 1. An optical modulator comprising: a first mesawaveguide extending in a first direction; and a second mesa waveguide,wherein the first mesa waveguide includes a p-type first semiconductorlayer disposed over a substrate, a core layer disposed over the firstsemiconductor layer, a p-type second semiconductor layer disposed overthe core layer, and an n-type third semiconductor layer disposed overthe core layer, the second semiconductor layer and the thirdsemiconductor layer are arranged adjacent to each other in the firstdirection, an electrode is disposed over the third semiconductor layer,and a joining surface between the second semiconductor layer and thethird semiconductor layer is inclined with respect to a surfaceorthogonal to the first direction.
 2. The optical modulator according toclaim 1, wherein the joining surface is inclined such that a length ofthe third semiconductor layer in the first direction is decreased in adirection from the electrode toward the core layer.
 3. The opticalmodulator according to claim 1, wherein a p-type dopant in the secondsemiconductor layer has a concentration of 1×10¹⁶ cm⁻³ or more.
 4. Theoptical modulator according to claim 1, wherein a p-type dopant in thesecond semiconductor layer has a concentration of 1×10¹⁷ cm⁻³ or less.5. The optical modulator according to claim 1, wherein the secondsemiconductor layer includes semiconductor material same assemiconductor material of the first semiconductor layer.
 6. The opticalmodulator according to claim 1, wherein the second semiconductor layerincludes InP doped with Zn.
 7. The optical modulator according to claim1, wherein a p-type dopant in the second semiconductor layer has aconcentration smaller than a concentration of a p-type dopant of thefirst semiconductor layer.
 8. The optical modulator according to claim1, wherein the second semiconductor layer has a thickness smaller than athickness of the first semiconductor layer.
 9. The optical modulatoraccording to claim 1, wherein a thickness of the second semiconductorlayer is from 1.0 μm to 3.0 μm.
 10. The optical modulator according toclaim 1, wherein the first mesa waveguide further comprises a fourthsemiconductor layer disposed over the second semiconductor layer. 11.The optical modulator according to claim 10, wherein the fourthsemiconductor layer includes a semiconductor material different from asemiconductor material of the second semiconductor layer.
 12. Theoptical modulator according to claim 1, wherein the joining surface isinclined by an angle θ with respect to a main surface of the substrate,the angle θ being from 50° to 60°.
 13. A method for producing an opticalmodulator including a first mesa waveguide extending in a firstdirection and a second mesa waveguide, the method comprising: forming asemiconductor stack over a substrate, the semiconductor stack includinga p-type first semiconductor layer disposed over the substrate, a corelayer disposed over the first semiconductor layer, and afirst-conductivity-type second semiconductor layer disposed over thecore layer; wet-etching the second semiconductor layer to form a recessin the second semiconductor layer, a length of the recess in the firstdirection being decreased in a direction from an upper end toward alower end of the recess; forming a second-conductivity-type thirdsemiconductor layer in the recess; etching the second semiconductorlayer, the third semiconductor layer, the core layer, and the firstsemiconductor layer to form the first mesa waveguide and the second mesawaveguide; and forming an electrode over the first mesa waveguide, theelectrode being formed over an n-type layer selected from the secondsemiconductor layer and the third semiconductor layer.
 14. The methodfor producing an optical modulator according to claim 13, wherein thethird semiconductor layer is the n-type layer.
 15. The method forproducing an optical modulator according to claim 13, wherein in theforming the third semiconductor layer, a length of an upper surface ofthe third semiconductor layer in the first direction is larger than alength of an upper surface of the second semiconductor layer in thefirst direction.